Eggs 101: The Anatomy of an Egg

byDavid EllisonDecember 12, 2012

Be it baked, boiled, coddled, fried, poached or scrambled, a perfectly prepared egg is the hallmark of a good cook. With this post we begin an on-going series called Eggs 101, in which we will address each of these basic cooking techniques in-depth. By the time we reach the end, you will have mastered each and every approach.

But before we get cooking, we need to start with some basics, and it doesn’t get any more basic than an anatomy lesson. No, you don’t need to know the difference between a chalazae and the thin albumen to make a decent omelet, but you can learn a lot about cooking an egg just from looking at it.

Shell

There aren’t too many culinary uses for an eggshell, but they do make the perfect storage container. Their rigid strength and semi-permeable membranes allow us to keep eggs fresh in the refrigerator for weeks. Shells are predominately calcium carbonate and have a thin outer membrane called a cuticle that forms a barrier against bacteria. The color of the shell is dependent on the breed of chicken that laid it and has no bearing on taste or nutritional value. If you want to put your old shells to good use, here is a list of 12 things you can do with them.

Shell Membranes

Between the shell and the egg white are the inner and outer membranes, which also act as barriers to bacteria.

Air Cell

The temperature of a freshly laid egg is about 105°F (41°C). As the egg cools, an air pocket forms between the shell membranes, usually at the broader, rounded end of the egg. This pocket, called the air cell, is most visible with hard-boiled eggs. As the egg ages, the cell grows larger.

Albumen

Known as the egg white, the albumen is 90% water and accounts for 69% of the egg’s weight. It provides nearly 4 grams of protein and has virtually no fat or cholesterol. Next the yolk, the albumen tastes bland, but it’s protein structures are what give meringues and soufflés their lift. There are two parts to the albumen. The white that immediately surrounds the yolk is the thick albumen, which is, in turn, surrounded by the thin albumen. When an egg is fresh the ratio of thick albumen to thin is 3:2. As the egg ages, the albumen spreads and the ratio drops to less than 1:1. The thickness of the inner albumen largely determines the grade of the egg.

Yolk

A powerhouse of nutrition, the yolk accounts for 31% of an eggs weight, 41% of its protein (~3 grams), and 100% of its fat (6g) and cholesterol (213mg). The yolk also provides iron, vitamins A and D as well as calcium, thiamine, and riboflavin. It also contains a lecithin compound which makes it a natural emulsifier for such things a mayonnaise. The yolk gets its yellowish orange color from a plant pigments in the hen’s feed. The sometimes-visible small indentation on the surface of the yolk is called the germinal disk and is where the sperm enters the egg. (Contrary to popular belief, a fertilized egg is no more nutritious than an unfertilized one.) Sometimes reddish-brown flecks are visible in the yolk. These are called blood or meat spots and are harmless.

Vitelline Membrane

A thin clear wall that holds the yolk together and makes it possible for us to separate the yolk, in tact, from the whites. Once the membrane is broken, you virtually have scrambled eggs. As the egg ages, the membrane weakens and the yolk spreads.

Chalazae

Pronounced “ka-lay-zee”, these small rope-like structures are part of the egg whites and tether the yolk to the inner shell membrane. It primary purpose is to keep the yolk centered. (Wouldn’t it be nice if we all had our own chalazae?) The more the prominent chalazae the fresher the egg.

Next Up in This Series: Visual Cues of a Fresh Egg

Sources:

McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004. Print.